The gains in storage capacity from increasing levels of pressure under which compressed gasses are held come at the cost of discharging large volumes in a desired short period of time. Many gas storage systems use multiple pressure vessels interconnected to manifolds. Discharging from a high pressure storage vessel (at for example 3500 psig) to a receiving terminal (rated for example at 1200 psig) can be accomplished by pressure equalization in a first stage of evacuation. The gas remaining in the container, which is now at the lower terminal pressure level, undergoes a second stage of evacuation by connection to a drawdown compression or low pressure manifold. As a result, the flow of compressed gas transported in multiple assemblies of pressure vessels at high pressures is frequently subject to a bottleneck of prolonged loading and unloading times.
In seeking to improve the evacuation of the contents of these storage vessels, proposed systems, such as that disclosed in Bishop U.S. Pat. No. 6,655,155, seek to displace the gas under full holding pressure using a displacement liquid in a manner similar to that used to move product from underground storage caverns. Given the gross volume of all storage containers, it is possible to purge all interconnected vessels by simultaneous displacement with an equal volume of liquid. However, such an equal volume would require a large shore mounted supply with recycle facilities in the case of marine transportation. Such a volume would be impractical to carry on board a ship and require inordinate amounts of motive power. In response to this problem, Bishop advocates a staged tier displacement system designed into the ship reducing a 200,000 bbls initial on board storage need to 50,000 bbls of on board storage.
Accordingly, it would be desirable to provide an improved evacuation system and method that facilitates the reduction of the amount of displacement liquid used for unloading compressed gas from a multi-vessel storage system and to improve evacuation times by displacement of the compressed gas contents in their entirety from the storage vessels.